According to the existing art, automatic transmissions, in particular for motor vehicles, usually encompass planetary gear sets that are shifted by means of frictional or shifting elements, for example clutches and brakes, and usually are connected to a initial movement element, for example a hydrodynamic torque converter or a fluid coupling, that is based on a slip effect and is selectably equipped with a lockup clutch.
A transmission of this kind is disclosed in EP 0 434 525 B1. It substantially encompasses an input drive shaft and an output drive shaft that are arranged parallel to one another; a double planetary gear set arranged concentrically with the output drive shaft; and five shifting elements in the form of three clutches and two brakes, the selectable locking of which, in respective pairs, determines the various gear ratios between the input drive shaft and output drive shaft. The transmission has a reduction gear set and two power paths, so that by selective paired engagement of the five shifting elements, six forward speeds are obtained.
For the first power path, two clutches are needed in order to transfer torque from the reduction gear set to two elements of the double planetary gear set. These are arranged in the power flow direction substantially after the reduction gear set in the direction of the double planetary gear set. For the second power path, a further clutch is provided that releasably connects it to a further element of the double planetary gear set. The clutches are arranged in such a way that the inner disc carrier constitutes the output drive.
A further planetary transmission of this kind is known, for example, from U.S. Pat. No. 4,070,927, the number of available forward gears being in each case one greater than the number of frictional or shifting elements. Each gear change between the forward gears is achieved by respectively switching one of the frictional or shifting elements in or out.
Also known, from DE 199 12 480 A1 of the present inventor, is an automatically shiftable motor vehicle transmission having three single-carrier planetary gear sets as well as three brakes and two clutches for selecting six forward gears and one reverse gear, in which the input drive shaft is connected directly to the sun gear of the second planetary gear set and is connectable via the first clutch to the sun gear of the first planetary gear set and/or via the second clutch to the carrier of the first planetary gear set. Additionally or alternatively, the sun gear of the first planetary gear set is connectable via the first brake to the housing of the transmission, and/or the carrier of the first planetary gear set is connectable via the second brake to the housing, and/or the sun gear of the third planetary gear set is connectable via the third brake to the housing.
Also described in EP 1 265 006 A2 is a six-speed automatic transmission of planetary design which also encompasses two brakes and two clutches, in which the individual gear ratios can be implemented by a combination of two shifting elements in each case.
Automatically shiftable vehicle transmissions of planetary design in general have already been described many times in the existing art, and are continuously being developed and improved. For example, these transmissions are intended to have a sufficient number of forward speeds as well as a reverse gear, as well as a conversion ratio that is highly suitable for motor vehicles, with a wide overall ratio spread and favorably spaced ratios. They should also make possible a high initial movement conversion ratio in the forward direction and contain a direct ratio, and be suitable for use in both passenger cars and commercial vehicles. These transmissions should also require little design complexity, in particular a small number of shifting elements; and should prevent double shifts upon sequential shifting, so that when shifts are made in defined speed groups, only one shifting element is changed in each case.
Transmissions of the kind described above generally have six forward speeds; a further, seventh speed would advantageously result in more convenience for the driver and in optimized fuel consumption. The seventh gear also optimally guarantees a sporty driving style.
It is the object of the present invention to propose a multiple-ratio transmission of the kind described above in which at least seven forward speeds can be implemented. It is intended, in the multiple-ratio transmission according to the present invention, that low torques act on the shifting elements and planetary gear sets; and that the rotation speeds of the shafts, shifting elements and planetary gear sets be minimized. The transmission according to the present invention is furthermore intended to be light in weight with compact dimensions. Any desired embodiment and arrangement of an initial movement element, and of the input drive and output drives, should also be possible.
At this point, and before entering the following descriptions, it must be noted that certain of the following descriptions refer to “minus transmissions” or “minus planetary transmission”, which are also commonly referred to in the relevant arts as “planetary-minus-gear sets”, and to “plus transmissions” or “plus planetary transmissions”, which are also commonly referred to as “plus-planetary-gear sets”. Although these terms are well known and commonly used terms of art in the relevant arts, a brief description of these terms will assist in understanding the full meaning of the following descriptions. In brief a planetary gearset or transmission is a planetary-minus-gear set or a planetary-plus-gear set when the planetary carrier is fixed and the sun gear is driven by an input speed in a defined direction of rotation. If the ring gear rotates, as a result of the sun gear rotation, in the same direction as the sun gear, then the planetary gear set is referred to as a planetary-plus-gear set or a plus-transmission. If the ring gear rotates, as a result of the sun gear rotation, in the direction opposite to the rotation of the sun gear, then the planetary gear set is referred to as a planetary-minus-gear set or a minus-transmission. In a typical implementation of a minus-transmission or a plus-transmission and, for example, the transmission or gear set is a double pinion type planetary gear set having a sun gear, a ring gear and two sets of planetary pinions wherein the first set of planetary pinions intermesh with the sun gear and the second set of planetary pinions while the second set of planetary pinions intermesh with the first set of planetary pinions and the ring gear. The terms, therefore, effectively describe a single pinion type planetary gear set having a sun gear, a ring gear and only one set of planetary pinions. It is well known, however, that there are several other types of planetary gear sets that can be referred to as a “planetary-plus-gear set” or a “planetary-minus-gear set. For example, a planetary gear set having a single sun gear, a single ring gear and an odd number of planetary pinion sets may be referred to as a “minus-transmission”, while a similar gear set having, however, an even number of planetary pinion sets, may be referred to as a “plus-transmission”.